It is the goal of this proposal to improve myeloablative regimens currently used to condition patients for allogeneic bone marrow transplantation (BMT). With the advent of recombinant DNA technology, a new set of genetic markers have become available. Using restriction fragment length polymorphisms (RFLP) as markers of bone marrow engraftment, a more comprehensive and quantitative evaluation of current conditioning regimens is feasible. In addition, RFLP analysis will enable comparisons of engraftment of progenitor cells (e.g., CFU-GEMM, CFU-E, CFU-C) with their more mature counterparts. Recently, T-cell depletion techniques have been implemented to decrease the incidence of graft-versus-host disease (GVHD), a frequent complication of BMT. Although T-cell depletion of donor marrow grafts has successfully reduced the incidence of GVHD in both histocompatible and histoincompatible BMT recipients, incomplete engraftment and graft rejection have emerged as significant complications of T-cell depletion. Therefore, this proposal will use the mouse as an experimental system to investigate parameters influencing bone marrow engraftment as a prelude to clinical trials of new myeloablative regimens, which will be required to improve the engraftment rate of patients receiving T-cell depleted donor bone marrow. RFLP analysis will be applied to a murine model of BMT to provide an accurate means of quantitating engraftment in multiple cell populations. Various radiological conditioning regimens employing single dose, split dose, fractionated and hyperfractionated total body irradiation with or without chemical conditioning, will be tested. In some animals, donor marrow will be depleted of T-cells by monoclonal antibody-ricin conjugates (immunotoxins: IT) a methodology in current usage in our institution for clinical BMT. Conditioning regimens will be tested in the context of T-cell depletion in a matched or mismatched murine setting. Evidence for cells implicated in graft rejection will be obtained from: 1)\RFLP analysis of individual cell populations (e.g., NK cells, macrophages); 2) conditioning of the host with IT directed at specific cell types; and 3) by changes in the engraftment status of mice receiving donor marrow depleted of specific cell types by IT treatment. These studies will provide information which will be of great clinical value leading to the development of new protocols for the University of Minnesota BMT program.